The present invention relates to a brake pressure control unit for a hydraulic dual circuit brake system of a road vehicle which is equipped with a hydraulic brake power booster which comprises a constantly supercharged hydraulic pressure accumulator and a brake valve connected to the accumulator. The accumulator inputs into a drive pressure space of the brake pressure control unit when the brake system is operated, a pressure which is proportional to an operating force and acts directly on a drive surface of a primary piston. This pressure imparts a longitudinal displacement on the primary piston and also a longitudinal displacement on a secondary piston in the sense of a brake pressure build up and a primary output pressure space and in a secondary output pressure space of the brake pressure control unit respectively. This vehicle has a static brake circuit connected to each of the primary and secondary output pressure spaces. The primary piston comprises an annular piston sealed from a bore of a casing, and a plunger piston arranged displaceably in a central bore of this annular piston and sealed from the annular piston. The plunger piston has a radial driving flange against which the annular piston is supportable by its end face toward the primary output pressure space. The annular piston and the plunger piston are pressurizable by the drive pressure inputtable into the drive pressure space and jointly displaceable in a sense of a brake pressure build up. The plunger piston remains displaceable by pedal power alone during a failure of an auxiliary pressure source.
Such a brake pressure control unit is known from German Published Unexamined Patent Application No. 3,347,618. In this known brake pressure control unit, the hydraulic auxiliary pressure supply source is also operated with brake fluid as a working medium. This has the disadvantage that a separate, electrically operated charge pump has to be provided for charging the pressure accumulator which is provided in association with the auxiliary pressure source and has to be kept constantly at high output pressure. This pump is switched on only intermittently, when the pressure in the pressure accumulator has dropped below a tolerable minimum value. The only intermittent operation of the accumulator charge pump is necessary since the lubricity of the usual brake fluid is not sufficient to ensure an adequate service life of an accumulator charge pump of the usual type operated with brake fluid, driven constantly, for example by the vehicle engine. In order to be able to utilize such a permanently driven accumulator charge pump, for example the pressure supply pump in a central hydraulic unit of the vehicle, for brake power boosting as well, it would be necessary to be able to operate the hydraulic booster circuit with a hydraulic oil of good lubricity. However, this would require a hydraulic media separation between the booster circuit and the primary output pressure space of the brake pressure control unit. Although such a media separation could be realized by creating in the known brake pressure control unit an additional separating space between an annular drive space and a follow-up space, which are delimited from each other by a piston flange of the annular piston of the primary piston of the known brake pressure control unit, and by also performing a corresponding measure with respect to the plunger piston of the known brake pressure control unit, that would however entail an increase in the overall length by at least one piston stroke, giving rise to unjustifiably adverse installation conditions in practice.
Therefore, an object of the present invention is to Provide a brake pressure control unit for a hydraulic dual circuit brake system which, at the same time as having a favorable small overall length, makes possible a brake power booster circuit both for an operation of the booster circuit with brake fluid and conventional hydraulic oil.
This and other objects are achieved according to preferred embodiments of the present invention by providing in a brake pressure control unit a first annular seal of the annular piston on the piston side, a flange facing the drive pressure space on which the first annular seal is arranged, and a second annular seal fixed to the casing and arranged on the end section facing the primary output pressure space of the casing bore guiding the annular piston. These first and second annular seals form an outer separating space in the shape of an annular gap. A third annular seal is arranged on an inner bore of the drive flange of the annular piston, while a fourth annular seal is arranged at the plunger piston and is offset in the direction of the primary output pressure space, these third and fourth annular seals sealing the annular piston and the plunger piston from each other.
The following are at least some of the advantages hereby achieved by the brake pressure control unit according to the invention:
The arrangement of an outer separating space, between the casing and an annular piston, which imparts the media separation between the hydraulic booster circuit and the primary output pressure space of the brake pressure control unit and whose axial extent varies with the displacement of the primary piston, has the effect that the extension of the brake pressure control unit by a distance corresponding to a maximum piston stroke, which is otherwise necessary for realization of such a separating space, is avoided. To this extent, an overall length corresponding to a "normal" tandem master cylinder is maintained and nevertheless an effective media separation between the drive pressure space of the brake power booster and the primary output pressure space of the brake pressure control unit is attained.
The brake power booster can be operated optionally with brake fluid or with a hydraulic oil which has an adequate lubricity for a pump permanently driven by the vehicle engine to be used as an accumulator charge pump. If hydraulic oil is used, the accumulator pressure is maintainable at the necessary value by a simple relief valve, since no electrical pressure switch for controlling an electric charge pump to be intermittently activated is required. The media separation on the plunger piston side by two annular seals, one of which is arranged at the annular piston and the other of which is arranged at the plunger piston, is generally, that is to say with intact brake power booster, fully adequate, since the annular piston and the plunger piston do not execute any relative movements with respect to each other.
By providing a plunger piston which is a step piston, a separating space is created between the annular piston and the plunger piston, which makes possible an even more effective media separation between the drive pressure space of the brake power booster and the primary output pressure space of the brake pressure control unit.
This effective media separation is enhanced in certain preferred embodiments in which the annular piston has an annular groove which communicates with the outer separating space via a cross-channel.
The inclusion of a second separating space, and the filling of the two separating spaces with a separating medium or the venting to atmosphere of these spaces, provides enhanced effectiveness of the media separation.
The design of a balancing valve according to certain preferred embodiments of the present invention, allows the necessary brake fluid balancing flow to take place between the brake fluid reservoir and the primary output pressure space of the brake pressure control unit, which otherwise can have the design of a tandem master cylinder of the conventional type.
According to features of certain preferred embodiments, the brake pressure control unit according to the invention is utilized in combination with electrically triggerable solenoid valves, which can be triggered by known control criteria, for an antilocking regulation, a propulsion regulation at the driven vehicle axle, and an increase which is appropriate for the axle load, and the brake pressure at the rear vehicle axle. This control is realizable for example by the brake slip at the rear axle constantly being regulated at a value associated with good braking acceleration. A particular advantage of the present invention is that a favorably small overall length of the brake pressure control unit is realizable even when utilizing these additional functions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.